This invention relates generally to detecting rotational acceleration and in particular embodiments to detecting rotational acceleration around at least two axes.
Acceleration can be measured using gyroscopes. However, gyroscopes tend to be expensive and are not always completely compatible with silicon based microelectronic products. In other words, gyroscopes cannot be easily integrated into a silicon format.
Thus, it is known to form accelerometers in silicon substrates. For example, cantilevered beam accelerometers may be incorporated into silicon substrates using etch processes to define the cantilevered beams. U.S. Pat. No. 5,101,669 describes a number of different techniques for forming silicon-based cantilevered beam accelerometers. Each cantilevered beam includes a flexure element and a proof mass at its unattached end. The response of the cantilevered beam to acceleration may be measured using capacitive or piezoresistive sensors. For example, a capacitive accelerometer may measure the displacement of the proof mass relative to the substrate by measuring the changing capacitance arising from the difference in the gap between the substrate and the proof mass.
However, when complex rotational acceleration is involved, the displacement of the cantilevered beam alone does not provide enough information to determine the precise nature of the sensed acceleration. Rotational acceleration has a direction, a magnitude and an axis of rotation or a pivot point. A cantilevered beam accelerometer can determine magnitude and direction but cannot determine the rotational center of the acceleration.
It desirable to know all three characteristics of rotational acceleration, for example in connection with blur compensating digital cameras. In digital cameras, the camera make shake relative to the object being imaged. The shaking may be due, for example, to the unsteadiness of the user's hands on a hand-held camera. As a result of the unsteadiness of the camera support relative to the imaged object, a variety of distortions may arise.
For example, referring to FIG. 1, displacements of the camera 10 along the axis X (which is parallel to the imaging axis of the camera 18), may result in changing the object distance. In effect, translation along the X axis alters the magnification of the image by modulating the camera to object distance. The sensitivity to this motion decreases as the object distance increases. For macro photographs, significant magnification modulation may arise from such motion. Conversely, a photograph of a scenic view may not change significantly. The same effect may occur along the Y and Z axes depicted in FIG. 1.
The sensitivity equation is different for rotation. Rotation in the YZ plane (i.e. rotation about the X axis) causes the image to streak in circles, the degree of streak varying between the center of rotation and maximum distance from the center in the image. Rotation in the XZ or XY planes causes apparent translation that gets worse as distance increases. For XZ and XY rotations, even the slightest shake disturbs the picture, particularly when the imaged object is far off in the distance.
Thus, various techniques have been considered to compensate for motion induced blurring. Some techniques attempt to electronically measure the motion by analyzing the captured image information. Other approaches attempt to measure the movement of the platform supporting the camera and to feed this information back to correct the image blurring.
However, all of these techniques suffer a variety of disadvantages. Predominantly, such techniques are not easily incorporated into the same silicon substrates which form the imaging arrays of the digital imaging systems. As a result, they add components and cost to the overall system.
Thus, there is a continuing need for an effective way locate the rotational center of rotational acceleration around more than one axis. In particular, in connection with digital imaging applications for example, it is desirable to provide acceleration measurement technology which may be incorporated into the same silicon substrates which form the imaging sensors.